Out With the Old
The planes of the world are, broadly speaking similar to those of the standard D&D pantheon, with some adjustment to the overall arrangement of the outer and inner planes. The Material, Feywild, Shadowfell, and Ethereal Planes are essentially the same.
The Inner Planes are less ordered. The natural state of the elemental is Elemental Chaos. The four elements are blended into a swirling storm of substance and energy. Matter shifts wildly between states, even shifting from one material to another. Gravity is mostly absent but can also shift chaotically, appearing and disappearing unpredictably. The Elemental Chaos is functionally identical to Limbo (see DMG p. 61). The stabilizing force of the Astral Sea has stabilized an immense portion of the chaos into the recognizable elemental planes.
The elemental planes are arranged in something like a ring or sphere, which is divided into distinct realms such as the Plane of Fire, the Plane of Water, the Plane of Earth and the Plane of Air. But these realms are not truly discrete. Air and Fire collide to form the Plane of Ash, which is in turn bordered by the Sea of Fire and the Sirocco Straits. Earth and water collide to form the Plane of Ooze, which is bordered by the Silt Flats and the Mud Hills. Fire and Earth collide to form the Plane of Magma which is bordered by the Fountains of Creation and the Furnaces. Air and Water collide to form the Plane of Ice which is bordered by the Sea of Ice and the Mistral Reach. Likewise, in a four-dimensional dodge collision that most planar scholars don’t like to think about too hard, Earth and Air collide to form the Plane of Dust and Fire and Water collide to form the Plane of Steam. The boundaries between the planes are to some degree arbitrary, as is the division between the elemental planes and the Elemental Chaos. “Below” the Chaos roils the Abyss, primal chaos corrupted by the Seed of Evil.
Celestials are the divine servants of the gods of good, and originate on most of the upper planes. Fiends are the corrupt spawn of the Seed of Evil, and come in three varieties. The Demons were spawned by The Chained God when he planted the Seed of Evil in the Abyss. The Devils were created when Asmodeus stole a shard of the Seed of Evil to create his Ruby Rod and slew He Who Was. And the Daemons, also known as Yugoloths were latter births of the abyss, born in realms where the corruption of the Abyss was adulterated by the stabilizing influence of the gods. Yugoloths are still most often born in the Abyss, but as cosmic mercenaries they have spread to countless other planes, where they have nativized.
Upper (Outer) Planes The Upper Planes are accessible via the Astral Sea
Exalted Immortality. The departed faithful of each god appear in a given Outer Plane, but not all who die are permitted an afterlife; only those who truly embodied a god’s ideals may be thus exalted. The rest pass on to the mysterious ultimate fate that awaits all life. The exalted enjoy a sort of immortality; so long as they are on their home planes, they cannot truly die. If slain, they return to life the following day, typically at dawn or sunset, but sometimes at other times. Over time the souls of the exalted fade, and merge with the essence of their god, adding to the strength and size of the plane and of their god. Thus do particularly virtuous (or vicious) souls find eternity in the paradise of their choosing.
Faithless and False. Those who did not live up to the ideals of their chosen gods typically simply pass beyond knowing. However, there are some exceptions to this. First, not all faithful declare a strong allegiance to a particular deity, and some are not terribly devout at all. While intentional devotion is helpful in selecting one’s afterlife, it is not actually necessary; if a soul is above all noble and just, then it might awaken after death under Bahamut’s care, even if she paid lip service to the cult of Tiamat or even completely without faith, while a vocal devotee of Pelor who was violent in cruel might find an afterlife in Chernoggar, or more likely none at all.
Outsiders. Due to the disruption of the Lattice of Heaven, not all slated for exaltation find it. Many instead appear on the “islands” drifting in their destination’s color sea, banned from actually entering it. Some believe that this is because they are unworthy of their final reward. Each has its own way of addressing or ignoring the problem of the outsiders, be it to ignore them, to poach them, to persecute them, or to seek a means of welcoming them into their god’s embrace. However, marked as they are as the children of a given deity, they are unable to join any other afterlife, either. An outsider who sets foot in an Outer Plane is teleported to a random location just outside that plane. Some camp on “their” planes’ color seas, petitioning for entry, while others become disaffected, and wander away, or are taken by one of the more predatory elements of the Silver Void. Some outsiders are faithless or false, and this leads many to believe that this exile is the fate of such souls, resulting in some harsh opinions about the true devotion of all outsiders.
The Exceptions. Pluton is an exception; as it once was the realm through which all the dead must pass, all the dead may enter the Gray waste, though this is not a fate that most outsiders would choose. Carceri is a cosmic prison; an outsider who manages to enter that realm is trapped as surely as anyone else who is imprisoned there. The Nine Hells are a corruption of the Lattice of Heaven. No matter how many souls the servants of Asmodeus might seduce or steal, no soul truly belongs to Hell, and thus no soul can be excluded either; whatever aberration of divine law prevents a deserving soul from entering other realms, it does not apply to Hell. Souls taken into the Nine Hells are used up and destroyed or transformed into the base material of new devils, not exalted.
The Lower (Inner) Planes: The Planes of Earth, Air, Fire, Water, Ice, Ooze, Ash, and Magma are more or less identical to their descriptions on pages 52-57 of the Dungeon Master’s Guide. Additional details of these planes are noted here.
The inner planes are more or less isolated from the Astral Sea, and few to no Astral portals lead to the inner planes. One typically reaches an inner plane via the Ethereal Plane.
Planar Waterways: Water is often a boundary between worlds. Pools and streams become portals to the Feywild and Shadowfell. The Astral Plane itself is often navigated as if it were a vast and infinite ocean. And then there are the rivers.